In Wheelock, on page 220, it says "in the following table, adverbs that do not follow the standard rules for forming adverbs from adjectives are highlighted". But male is highlighted, implying that it is an irregular formation from malus. But it seem to follow the rules to me: add -e to the stem mal-.
I can see the irregularities in all the other highlighted forms, - is the highlighting simply reminding readers that these are irregular adjectives? or is it a misprint, or have I missed something (again!)?

I haven't seen Wheelock's illustrious work, but it could well be the case that his inclusion of male as "irregular" is based upon malus' adoption of irregular comparitive and superlative adverbs, peius and pessime respectively.

Ta very much, - yes we've covered some of the irregular adjectives .. immo adjectives with irregular comparatives and superlatives, but that paragraph implied that male was of itself an irregular formation from adjective to adverb.
Hmmm ... illustrious... not the word I'd have used.
Phil

Perhaps it is marked because of the short final e? Most adverbs with this formation end in a long vowel, but Wheelock does not usually point subtleties like that out, so this is unusual of the book to say the least. He could also have been referring to the entire comparative system, as whiteoctave said, but if so then the table isn't very consistent.